In Japan, most paging systems transmit pages more than once to improve paging performance. In general, the multiple transmissions of a message are spaced significantly apart in time. Since a pager on a repeat system is given multiple opportunities to receive the same message in potentially different RF environments (due to the variance of field strength over time and the time difference of transmissions), the pager will gain paging sensitivity. This concept is generally known as time-diversity. Depending on a selective call receiver's (or pager's) knowledge of the timing of a message's repeats, the pager can "build" a message. Messages can be "built" at various levels within a messaging protocol structure or scheme, including at the page level, at the codeword level, or at the bit level. If a pager knows the timing (128 frames per cycle in Motorola's FLEX.TM. high speed paging protocol) in which it expects the repeats of a page, it can "build" the message at the page level by selecting the best message of all the repeats it receives. Sequential lockout is used to eliminate multiple alerts for the same message when a preceding message in a time diversity system is received error free. For instance, a pager in a POCSAG system where the message is repeated three times would ignore the second and third repeats if the message was decoded error free on the first repeat or ignore just the third repeat if the message was decoded on the second repeat.
If a pager knows the time frame in which it expects the repeats of a page and can be guaranteed that pages received during that time frame are repeats, it can "build" the message at the codeword level by combining the best message codewords of all the repeats it receives. This type of "building" is referred to as message codeword combining (MCWC) and is currently used in the NTT 1200 signaling scheme in Japan. Finally, if a pager knows the exact time it expects the repeat of a page, it can "build" the page at the codeword level by combining the best of all codewords of all the repeats. Alternatively, the pager could "build" the page at the bit level by combining the best bits of all the codewords (including address and/or vectors) of all the repeats. These two types of "building" are currently supported by FLEX.TM.-TD, the version of FLEX.TM. used in Japan. They are referred to as codeword combining (CWC) and bit combining (BC).
In any time diversity system, there is typically a latency involved in transmitting, receiving and decoding an entire message because of the nature of the repeated messages. When a message is sent that is longer than a predetermined size (in the case of FLEX.TM., longer than a frame, for instance), then the time diversity system must further accommodate and account for the sectioning of the message into fragments. Thus, fragmenting of messages causes further latency in a time diversity system. The examples in FIGS. 2 and 3 are illustrative of the latency problems in both non-Time Diversity and Time-Diversity systems respectively.
Initiating and terminating message fragments in a time diversity system is also a problem where, as in FIG. 1, a long message 100 is sectioned into six different fragments, A, B, C, D, E and F. In the case where the message consists of displayable characters and each character is represented by a fixed number of bits, it is desirable to display each fragment independently. In the case where the number of bits used to represent each character is a submultiple of the number of information bits used by each "over the air" code word, the character positions within each code word is constant and the contents of a fragment may be displayed. In cases where the fragments are of fixed length it is possible to determine character positions by knowing the fragment number or by knowing the character position prior to missed fragments and by knowing the number of fragments missed. When the number of bits used to represent a character does not divide evenly into the number of information bits per code word and the fragment size is chosen dynamically to fit channel capacity criteria, the character boundaries will precess through code words. When a fragment is missed, the character positions in all following fragments can not be determined (in the general case) and the remaining message fragments can not be displayed.
Consider the message in the vicinity of a fragment boundary to be represented by 8 bit characters as shown below:
______________________________________ . . ., 12345678, 12345678, 12345678, 12345678, 12345678, 12345678 , 12345678, 12345678, . . . ______________________________________
If the message above were split into two fragments using (31,21) BCH code plus even parity formatting as defined for FLEX and POCSAG into fragments N and N+1, then fragment N would appear as:
______________________________________ . . ., 12345678, 1 2345678, 12345678, 123456 ______________________________________
and fragment N+1 would appear as:
______________________________________ 78, 12345678, 12345678, 123 45678, 12345678, 123 . . . ______________________________________
Thus, in this example, when a fragment is missed in decoding, the character positions in all following fragments can not be determined (in the general case) and the remaining message fragments can not be displayed. Therefore, there is a need for initiating and terminating message fragments in a time diversity system in a form that would allow a pager to decode the remaining fragments where a prior fragment is missed.